The invention relates to radiography apparatus. The apparatus comprises a radiation source with an adjustment mechanism for generating radiation having at least two different energy spectra. It also comprises a radiation detector having an ionization chamber with a high voltage electrode, a signal electrode and an entrance window.
A radiography apparatus of this kind is known from U.S. Pat. No. 3,974,386. For many applications a clear distinction between the images formed by means of radiation having different energy spectra is very desirable. If the different energy spectra are obtained by the application of different high voltages to an X-ray tube as described in that Patent, the photons energy in the respective spectra overlap substantially. If an attempt is made to limit the overlap by more thorough filtering, either (i) the primary radiation intensity generated by the source would have to be made very high, which would reduce the life of the source or would necessitate the use of an expensive and heavy source, or (ii) one of the signals would become so weak that only very noisy images would be formed.